1. Field of the Invention
The invention relates generally to processing of single- or multi-channel non-stationary waveforms that occur, for example, in seismic exploration, speech, radar, sonar, radio, space communication, electron microscopy and medical remote sensing fields.
In particular, methods of the invention estimate the time-space varying frequency and wavenumber spectra, time delay between signals, dispersion, attenuation, dip and azimuth, curvature and higher-order reflection surfaces, and faults from signals in a 3D seismic volume. Further, this invention includes an apparatus to visualize and interpret results produced by the estimation methods.
2. Description of the Related Art
In seismic exploration for oil and gas the Earth's subsurface is illuminated by an acoustic source disposed at or near the surface of the Earth. Acoustic waves propagate down into the Earth and are reflected back to the surface from a sequence of layer interfaces. Reflected waves are recorded by an array of receivers typically arranged in a linear, 2 dimensional pattern or 3 dimensional pattern. The receivers can be positioned on the Earth's surface, on the ocean bottom, towed near the water surface, disposed inside a well or arranged in any geometrical pattern in two or three dimensions. In a seismic survey the combination of source and receiver array is typically relocated at a multiplicity of overlapping areas in order to uniformly illuminate the subsurface in a region of interest.
The seismic “wave” starts from what can be approximated as a point source. The wavefront from the acoustic source starts approximately as a sphere. The spherical shape becomes distorted and the radius of the sphere expands as the wavefront propagates down through layers in the subsurface to a reflector, whereupon the wavefront returns to the receivers. Reflected wavefronts from many reflectors in the Earth's subsurface overlap when they reach the receivers.
Each receiver output can be digitally sampled at equal time intervals. A ‘trace’ is a series of digital sample values indexed with respect to time. A trace typically comprises overlapping wave signals arriving at the receiver from different directions and sometime different sources, for example, another survey being concurrently shot in the neighborhood.
A ‘shot gather’ is a set of traces from multiple receiver locations that are displayed horizontally in their linear- or 2D-receiver positions relative to the acoustic source. Signals reflected from a single reflector in a shot gather typically align along a hyperbola in the case where the signals are acquired by a linear receiver array. Such signals align over a hyperboloid surface on a ‘shot gather’ when the signals are acquired by a 2D receiver array. Such alignments are referred to as alignment surfaces of coherent signals. From here onward when referring to a “surface” on a 2D receiver array, it is automatically implied to mean an alignment along a curve if the 2D receiver array were replaced with a linear receiver array.
The curvature of the alignment surface contains information about the velocity of the medium traversed by the wavefront. Further, the curvature decreases with the down-up (two-way) acoustic travel time to the reflector. Consequently, the trace-to-trace time delay between coherent signals varies with time. This requires that the time delay should be estimated over short signal windows. Further, the amplitude and phase spectra of signals within a short time window of the hyperboloid surface contain the “fingerprint” of the layer interface in a small volume around the reflection point.
Aside from reflected signals, the ‘shot gather’ also contains organized and random noises. Organized noise or ‘unwanted signal’ generally has a planar or quadratic surface alignment across a 2D array ‘shot gather’. The amplitude and phase spectra and alignment surface of the noise may be significantly different from those of signals reflected from layer interfaces. Signals must be analyzed along their alignments in order to characterize them and subsequently separate or suppress them.
A feature of modern seismic surveys is multi-fold recording. A common subsurface area of the Earth is surveyed by many receivers that are separated from sources by different offset distances and along different azimuths. Shot gathers are sorted such that their source-to-receiver mid-points fall into common “bins” in a grid pattern. Sorted gathers are called common mid-point (CMP) bin gathers. CMP gathers are analyzed and processed in various ways in a data processing center. Such processes include noise suppression, and velocity analysis that exploits the curvature on a CMP gather.
In land seismic surveys the velocities and thicknesses of near-surface layers vary rapidly over an area. This causes uneven time delays in the alignment of reflected signals from trace to trace. A process called ‘static correction’ is applied to each trace. It is the time shift that compensates for inhomogeneity of near-surface layers and sets the origin of each trace to a common datum. A further complication arises from surface waves that are dispersive, that is different frequencies travel with different speeds. Thus frequencies are delayed by different amounts as they travel between receivers. The term ‘normal dispersion’ is used when lower frequencies travel faster than higher frequencies. The term ‘anomalous dispersion’ is used for the opposite case of lower frequencies traveling slower than higher frequencies.
In order to identify static corrections or parameters of a reflection alignment surface it is necessary to estimate of trace-to-trace time delay between signals in a short time window. The study of dispersion phenomenon, however, requires that time delays must be estimated over a range of frequencies. In this invention we use the term ‘time delay spectrum’ to describe time delay that varies with frequency. The time delay spectrum is computed from short time phase spectra of two signals.
Attenuation is a phenomenon where the energy loss in a signal due to absorption increases with frequency as it travels through the Earth. Its effect is computed from short time amplitude spectra of a signal that is reflected from separate interfaces in the Earth.
An important process applied to seismic traces is called migration. Migration in effect “propagates” the wavefront downward from the receiver into the Earth's subsurface. In typical migration processing, the wavefront is approximated by a sequence of plane waves. This requires determination of lateral derivatives, or dip and azimuth, of wavefronts at each point of signal alignment. The migration process back tracks the wave path and stops at the point of reflection. Signals from different source-receiver geometries that are reflected from the common subsurface area are summed together into an image signal. A vertical collection of image signals as a function of time is an “image trace.” On an image trace, signals from reflectors with different dips and azimuths that are observed from various source-receiver separations are positioned vertically below their surface location at their two-way times. Further, the migration process reduces the volume of data by a ‘fold’ equal to an average number of repeated reflection points in a common bin. Migrated data is typically interpreted on a workstation equipped to handle 3D volumes, color displays and animation. The interpretation process also requires estimation of time delays, dip and azimuth, and curvature. In addition, the process requires the detection and mapping of discontinuities such as faults or buried channels.
In summary, there is a need for time-frequency spectral analysis, estimation of time delay spectrum, dispersion, attenuation, dip, azimuth and curvature of reflections plus detection and mapping of faults. Reviewed below are prior art methods that perform some of these operations.